1. Field of the Invention
This invention relates to a Potter-Bucky device for use in a radiation image recording apparatus, and more particularly to a Potter-Bucky device which cannot transmit adverse vibrations to other parts of an apparatus in which it is employed.
2. Description of the Prior Art
There has been in wide use a radiation image recording apparatus in which a radiation image of an object is recorded on an X-ray film by exposing the X-ray film to radiation such as X-rays which have passed through the object.
Further, there has been known a radiation image recording and reproducing system in which a radiation image is recorded and reproduced by the use of a stimulable phosphor instead of the X-ray film. When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store part of the energy of the radiation. Then when the phosphor which has been exposed to radiation is exposed to a stimulating ray, such as a laser beam, light is emitted from the phosphor in proportion to the amount of radiation energy which was stored by the phosphor. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In a radiation image recording and reproducing system, a radiation image of an object such as the human body is recorded and reproduced by the use of such a stimulable phosphor. Specifically, as disclosed, for instance, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, a recording medium bearing thereon a stimulable phosphor layer is first exposed to radiation which has passed through an object in order to store a radiation image of the object in the stimulable phosphor layer, and then the stimulable phosphor layer is two-dimensionally scanned with a stimulating ray which causes it to emit light in a pattern corresponding to the stored radiation image. The light emitted from the stimulable phosphor layer upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is processed to reproduce the radiation image as a visible image on a recording medium such as a photosensitive material, a display system such as a CRT, or the like.
This system is advantageous over conventional radiography which uses X-ray film in that a radiation image can be recorded over a much wider radiation energy exposure range. That is, it has been found that the intensity of light emitted from the stimulable phosphor upon stimulation thereof after it is exposed to radiation remains proportional to the energy of the radiation to which it was exposed for a very wide radiation energy intensity range. Accordingly, even if the energy intensity range of the radiation to which the stimulable phosphor is exposed varies substantially due to changes in the recording conditions, a visible radiation image independent of variations in the radiation energy intensity range can be obtained by choosing an appropriate gain when converting the light emitted from the phosphor into an electric signal.
In radiation image recording systems in which X-ray film or a stimulable phosphor layer (both generically referred to as an image recording medium) is exposed to radiation which has passed through an object so as to record a radiation image of the object, a grid is sometimes disposed between the object and the image recording medium. That is, when a relatively thick part of an object, such as the chest of a human body, is radiographed, diffused radiation emitted from the object upon radiographing can deteriorate the quality of the radiation image obtained. In such cases, a grid device for absorbing the diffused radiation should be provided between the object and the recording medium. As is well known, a grid device comprises lead foils or the like arranged in parallel or in a grid and is disposed so as to overlap with the whole image recording area of the image recording medium.
When such a grid device is kept stationary during the recording of a radiation image, fine stripes corresponding to the foils of the grid are projected onto the recording medium together with the radiation image of the object. Since the stripes are very fine, the stripes are almost invisible when the radiation image is recorded on X-ray film. However, when the stimulable phosphor layer is used to record the radiation image, the stripes are visible as moire fringes since the stimulable phosphor is more sensitive than the X-ray film and the radiation image stored in the stimulable phosphor layer is read out by scanning the stimulable phosphor layer with a light beam at very fine pitches. Particularly, in radiation image recording systems in which a stimulable phosphor layer is utilized and subtraction processing of image signals is involved, such as the systems disclosed in U.S. Pat. Nos. 4,710,875, 4,590,517 and Japanese Unexamined Patent Publication No. 58(1983)-163339, and the like, the stimulable phosphor layers storing therein different radiation images such as ones recorded before and after the infusion of a contrast medium, or digital image signal tapes bearing thereon image signals read out from the stimulable phosphor layers are subjected to translation processing and/or rotation processing in order to correct fluctuation in the position of the radiation images recorded on the stimulable phosphor layers. The fine stripes recorded on the stimulable phosphor layers interfere with each other during translation and rotation processing in such a way that they produce moire fringes in the finally reproduced image, thereby very adversely affecting diagnoses based on the reproduced image.
As disclosed in EP-0114978, there has been proposed a Potter-Bucky device having a driving means for driving the grid back and forth at a high speed in parallel to the stimulable phosphor layer. This arrangement prevents part of the recording medium from being shielded by the foils of the grid during image recording, thereby preventing the formation of stripes on the recording medium and the production of moire fringes in the radiation image which has been read out, even if the recording medium uses a stimulable phosphor layer.
However there is a problem in that, though the Potter-Bucky device can prevent production of the moire fringes, it is apt to transmit vibrations to other parts of the system due to the movement of the grid. Particularly, in the case of a so-called built-in type radiation image recording and reproducing system, in which an image recording device, an image read-out device and an erasing device are incorporated into a single unit and the recording medium having the stimulable phosphor layer is conveyed or circulated through the system, when vibrations are transmitted from the Potter-Bucky device in the image recording device to the image read-out device, if the image read-out device is carrying out the image read-out operation, the accuracy of the read-out operation deteriorates.